A catalyst comprising a zirconium compound and aluminoxane has been recently proposed as an olefin polymerization catalyst. For instance, catalyst systems in which a metallocene compound having a pentadienyl group (e.g., cyclopentadienyl group), an alkyl group and/or a halogen atom as a ligand is used in combination with aluminoxane are described in Japanese Patent Laid-Open Publications No. 19309/1983, No. 35006/1985, No. 35007/1985, No. 35008/1985, No. 130314/1986 and 41303/1990. Also described is that these catalyst systems are highly active in the polymerization of .alpha.-olefin.
Other than such catalyst systems using a metallocene compound and aluminoxane in combination as mentioned above, those using a metallocene compound and an organoboron compound in combination are known to show activities in the polymerization of .alpha.-olefin ("Macromolecules" 1993, 26, 3239; "J. Am. Chem. Soc." 1991, 113, 3623).
However, if an olefin is polymerized by the use of the catalyst systems stated above (catalyst systems using one kind of a transition metal compound), a polyolefin having an extremely narrow molecular weight distribution (Mw/Mn) of about 2 is produced. In this polyolefin, therefore, there are eagerly desired the improvement in moldability and in surface appearance of the molded articles.
To cope with those problems, processes for preparing polymers of a wide molecular weight distribution by the use of catalyst systems in which plural kinds of transition metal compounds and assisting catalyst components are used have been proposed and described in, for example, Japanese Patent 35006/1985, No. 35008/1985, No. 501369/1988, No. 283206/1992 and No. 230135/1993.
In these processes, however, polymers having different properties are produced from each transition metal compound catalyst components, so that the reaction process is intricate, and the molecular weight distribution of the resulting polymer is not always controlled easily. Therefore, in order to obtain a polymer of desired molecular weight distribution, complicated operations are required.
Accordingly, there is eagerly desired the advent of an olefin polymerization catalyst by the use of which a polymer having a wide molecular weight distribution can be obtained in spite of the catalyst system using one kind of a transition metal compound and an .alpha.-olefin can be polymerized with high polymerization activity. There is also desired the advent of a process for olefin polymerization using said olefin polymerization catalyst.
Moreover, the catalyst system in which the metallocene compound is combined with aluminoxane or the organoboron compound have been generally known to exhibit highest polymerization activity at a polymerization temperature of 60.degree.-80.degree. C. However, the molecular weight of polymers produced at such temperature is still insufficient for molding many kinds of products. While, in such case, the molecular weight of the polymer obtained can be improved by carrying out the polymerization at a temperature lower than the usual polymerization temperature, a sufficient polymerization activity cannot be obtained. Accordingly, there is eagerly desired the advent of an olefin polymerization catalyst which exhibits high polymerization activity at low polymerization temperature and can give polymers having high molecular weight, and a process for polymerizing olefin using said olefin polymerization catalyst.
The inventors have made extensive and intensive studies in order to cope with the above problems associated with the above prior art. In result, it has unexpectedly been found that these problems can be solved by using, in an olefin polymerization catalyst, a transition metal compound having at least two transition metals in one molecule, in which at least one of the transition metals bonds to a ligand having a cyclopentadienyl skeleton, at least one of the transition metal is selected from the group consisting of Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and lanthanoid metals, and one of the other is the specific transition metal selected from the same as or different from said metals.